Melanocytoma of the optic disc is an ophthalmic tumor that arises from melanocytes and is a variant of the melanocytic nevus. This pigmented lesion occurs on the optic disc and often extends into the peripapillary retina and choroid. Before the 1960s, melanocytoma of the optic disc was thought to be a malignant neoplasm. Numerous case reports described the occurrence of a small, mildly elevated, pigmented lesion on the optic disc with peripapillary extension, for which eyes were enucleated because of concern about possible choroidal melanoma. In 1962, Zimmerman named this lesion melanocytoma and declared it to be a benign tumor.1 Given that optic nerve melanocytomas are composed of intensely pigmented, round or oval nevus cells with benign features and copious quantities of cytoplasm, a more appropriate name for the lesion may be magnocellular nevus of the optic disc. However, by convention, the term melanocytoma is still in use today.

Ophthalmoscopic Features and Diagnosis

Melanocytoma of the optic disc appears as a dark brown or black lesion. While some melanocytomas are confined to the disc, most cases involve the adjacent retina and choroid (Fig. 1). The choroidal component looks identical to a juxtapapillary choroidal nevus, while the retinal component is usually black with a feathery margin. Melanocytomas are typically small, averaging 2 mm in diameter and 1 mm in thickness, and are almost always unilateral. Associated findings on exam may include optic nerve edema, small retinal hemorrhages, retinal edema, retinal exudates, subretinal fluid and vitreous seeding. The mean age at detection is 50 years old, and the distribution among races is equal.2

DDx. Before the diagnosis of melanocytoma is made, it is critical to rule out a juxtapapillary choroidal melanoma. While melanoma can be juxtapapillary or can occur as a primary lesion at the optic disc, both these occurrences are quite rare. Features typical of choroidal melanoma are a thickness of more than 1.5 mm, subretinal fluid and the presence of orange pigment. Given the typical appearance of a melanocytoma, the diagnosis is generally straightforward and can be made with ophthalmoscopy alone. However, if an optic disc melanocytoma has atypical features, choroidal melanoma can only be ruled out after the patient has been followed over time and no changes suggestive of a melanoma have been observed. Other considerations in the differential diagnosis are choroidal nevus, hyperplasia of the retinal pigment epithelium, adenoma of the retinal pigment epithelium and metastatic melanoma of the optic disc.2

Ancillary testing for the diagnosis of melanocytoma adds little to dilated ophthalmoscopy. Considering the relatively small size of the lesion, ultrasonography, CT and MRI are all of little value. However, ancillary testing can help to better quantify the lesion and diagnose any sequelae. In some cases, fluorescein angiography can be used to better demarcate the lesion, which is typically hypofluorescent in all phases unless there is associated subretinal fluid or optic disc edema. Recently, OCT has been used to evaluate optic disc melanocytoma. Characteristic features of optic disc melanocytoma on OCT include a gradual transition from normal retina into nodular tumor, a bright anterior border layer and dense shadowing associated with no internal detail (Fig. 4).3 OCT can be useful in identifying subretinal fluid associated with the lesion.

Visual Consequences

While its cells show benign features, melanocytoma of the optic disc is not always benign visually. Even though the lesion is usually asymptomatic, visual field defects and vision loss are both possible.

Visual field defects. Most melanocytomas cause visual field defects; up to 90 percent of eyes with optic disc melanocytoma will have some defect on perimetry. However, the defects are rarely symptomatic.4 Most of these visual field defects manifest as an enlarged blind spot, presumably due to tumor extension past the disc margin. Some melanocytomas can cause nerve fiber bundle defects that result from compression at the optic disc.2

Visual acuity loss. Most melanocytomas do not cause loss of visual acuity, but mild vision loss can be seen in about 25 percent of affected eyes.5 The main cause of associated vision loss is retinal exudation involving the macula. Risk factors for vision loss include retinal extension of the melanocytoma and subretinal fluid. Severe vision loss is rare but can occur secondary to central retinal vein occlusion, spontaneous tumor necrosis or even malignant transformation.5 While the fundus findings of CRVO are usually straightforward, spontaneous tumor necrosis can result in neuroretinitis or seeding of tumor cells into the vitreous.

What Is the Chance of Malignancy?

The transformation of optic nerve melanocytoma to malignant melanoma has been documented in 1 to 2 percent of cases.5 If there is progressive growth or extensive involvement of the optic disc along with vision loss, then malignant transformation should be considered, although 10 to 15 percent of melanocytomas will have subtle enlargement during follow-up. An initial tumor thickness of 1.5 mm or more is a risk factor for growth.5

Management and Follow-Up

At the time of diagnosis, fundus photos should be taken and visual field testing should be performed. Then patients should be followed annually with dilated ophthalmoscopy and fundus photography. Visual field testing should be repeated if there is suspicion of tumor growth. In addition, OCT can be used both to document the extent of the lesion at diagnosis and to track any progression that may not be apparent with ophthalmoscopy alone.

Vision loss accompanied by significant growth of the tumor at any point must raise concerns for malignant transformation or initial misdiagnosis. If an atypical melanoma has not been ruled out, then the patient should be followed much more frequently to determine whether the lesion is growing.

When severe vision loss does occur, suspicion of malignant transformation is heightened, and enucleation must be considered. Extensive involvement of the disc and progressive growth of the pigmented lesion concurrent with vision loss are particularly worrisome for malignancy. However, if the vision loss is a result of ischemic necrosis, the recovery of some vision may be possible with observation alone.

Conclusion

An unknown and previously undocumented pigmented lesion of the optic disc and peripapillary area is cause for serious concern. However, knowing the key ophthalmoscopic features of an optic disc melanocytoma can help in diagnosing this lesion and differentiating it from choroidal melanoma.
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